The present invention relates to internal combustion engines, and more particularly to a control system for calculating minimum valve lift for an internal combustion engine.
Intake valves control the air/fuel mixture into cylinders of an internal combustion engine. Exhaust valves control gases exiting the cylinders of an internal combustion engine. Cams lobes on a camshaft push against the valves to open the valves as the camshaft rotates. Springs on the valves return the valves to a closed position. The timing, duration and degree of the opening or xe2x80x9cvalve liftxe2x80x9d of the valves impacts the performance of the engine.
As the camshaft spins, the cam lobes open and close the intake and exhaust valves in time with the motion of the piston. There is a direct relationship between the shape of the cam lobes and the way that the engine performs at different speeds. When running at low speeds, the cam lobes should ideally be shaped to open as the piston starts moving downward in the intake stroke. The intake valve closes as the piston bottoms out and then, following compression combustion and expansion strokes, the exhaust valve opens. The exhaust valve closes as the piston completes the exhaust stroke.
At higher RPMs, however, this configuration for the camshaft lobes does not work as well. If the engine is running at 4,000 RPM, the valves are opening and closing 33 times every second. At this speed, the piston is moving very quickly. The air/fuel mixture rushing into the cylinder is also moving very quickly. When the intake valve opens and the piston starts the intake stroke, the air/fuel mixture in the intake runner starts to accelerate into the cylinder. By the time that the piston reaches the bottom of its intake stroke, the air/fuel is moving at a high speed. If the intake valve is shut quickly, all of the air/fuel stops and does not enter the cylinder. By leaving the intake valve open a little longer, the momentum of the fast-moving air/fuel continues to force air/fuel into the cylinder as the piston starts its compression stroke. The faster the engine goes, the faster the air/fuel moves and the longer the intake valve should stay open. The valve should also be opened wider at higher speeds. This parameter, called valve lift, is governed by the cam lobe profile.
VTEC (Variable Valve Timing and Lift Electronic Control) by Honda is an electronic and mechanical system that allows the engine to have multiple camshafts. VTEC engines have an extra intake cam lobe with a rocker that follows the extra intake cam profile. The profile on the extra intake cam keeps the intake valve open longer than the other cam profile. At low engine speeds, the valves move in accordance with the standard cam profile and the extra rocker is not connected to any valves. At high engine speeds, a pin locks the extra rocker to the two standard rockers that activate the two intake valves.
Other engines phase the valve timing. This does not change the valve duration; instead, the entire valve event is advanced or retarded. This is done by rotating the camshaft ahead a few degrees. If the intake valves normally open at 10 degrees before top dead center (TDC) and close at 190 degrees after TDC, the total duration is 200 degrees. The opening and closing times are shifted using a mechanism that rotates the cam. For example, the valve might open at 10 degrees after TDC and close at 210 degrees after TDC. Closing the valve 20 degrees later improves performance. However, it would be better to increase the duration that the intake valve is open.
The camshafts on some Ferrari engines are cut with a three-dimensional profile that varies along the length of the cam lobe. At one end of the cam lobe is the least aggressive cam profile, and at the other end is the most aggressive. The shape of the cam smoothly blends these two profiles together. A mechanism can slide the whole camshaft laterally so that the valve engages different parts of the cam. The shaft spins just like a regular camshaft, but by gradually sliding the camshaft laterally as the engine speed and load increases, the valve timing can be optimized.
Several engine manufacturers are experimenting with systems that would allow infinitely variable valve timing lift and duration. For example, each valve has an actuator. A computer controls the opening and closing of the intake and exhaust valves. These engines do not need a camshaft. With this type of engine control system, the maximum engine performance minimum emission output, maximum efficiency or some balanced combination of all three can theoretically be provided at every engine speed and load. The computer controller, however, must have an algorithm for valve lift that balances the energy consumption of the valve actuation system with the optimum thermo-dynamics of the engine. The calculations must also be computationally feasible by an engine controller at the anticipated calculation rates.
A method and apparatus according to the invention commands valve lift of an intake valve for a cylinder of an internal combustion engine. A first desired pressure ratio (Pcyl/Pint) is selected for minimum induction pumping losses. A maximum cylinder flow demand is calculated. Intake valve lift is calculated by matching the effective flow capacity through intake valves of the engine to the maximum cylinder demand. The intake valve is actuated based on the calculated valve lift.
In another aspect of the invention, method and apparatus according to the invention commands valve lift of an exhaust valve for a cylinder of an internal combustion engine. A second desired pressure ratio (Pexh/Pcyl) for minimum exhaust pumping losses is selected. A maximum cylinder demand is calculated. Exhaust valve lift is calculated by matching an effective flow capacity through the exhaust valves of the engine to the maximum cylinder demand. The exhaust valve is actuated based on the calculated exhaust valve lift.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.